1. Field of the Invention
The present invention relates to controlling a high voltage discharge lamp which is preferably used particularly in a vehicle headlight.
2. Description of Related Art
A device for turning on a high voltage discharge lamp (lamp) applied to a headlight for a vehicle is proposed in JP-A 9-180888 (U.S. Ser. No. 08/756,556) or JP-A 9-306684.
In the proposed device, as shown in FIG. 26, the voltage of a vehicular battery 300 is transformed into high voltage by a primary transformer winding 302a and a secondary transformer winding 302b installed to the side of a starting circuit 301 having a capacitor 303 that is charged by the transformed high voltage. Further, when the charge voltage of capacitor 303 reaches a set voltage or higher, discharge is carried out at a discharge gap 304, the charge voltage of the capacitor 303 is further increased by the discharge through transformer windings 305a, 305b and high voltage is applied to a lamp 306. Thereby, lamp 306 is turned on.
Further, lamp 306 is maintained turned on by supplying predetermined power from vehicular battery 300 on the primary side to a secondary side transformer winding 302c installed for use in a direct current power source circuit 307 separately from starting circuit 301. At this occasion, in order to turn on lamp 306 by alternating current, the direction of discharge current is alternately inverted by inverter circuit 310 having an H-bridge circuit 309 comprising four MOS transistors 309a-309d.
Further, primary current I flowing in the primary side transformer 302a is controlled by MOS transistor 311 for the primary which is a semiconductor switching element. On/off switching of the MOS transistor 311 is controlled by a PWM control circuit 312. The PWM control circuit 312 carries out a duty-cycle control based on lamp voltage VL applied to lamp 306 and lamp current IL flowing in lamp 306 and controls power supply to the secondary side in accordance with a condition (for example, electrode temperature) of lamp 306.
However, there occurs a problem of limited life of gap 304 since gap 304 is used as a switching element for turning on lamp 306.
For example, when the gap interval is shortened by foreign matter adhering to gap 304, there occurs a problem of lessening turn-on performance of lamp 306 since the discharge is carried out at gap 304 before a predetermined discharge voltage is achieved since the discharge voltage is lowered. Meanwhile, when the gap interval is lengthened by wear of gap 304, there occurs a problem where the gap discharge cannot be carried out since the discharge voltage is made too high.
Further, there occurs a problem where seal gas inside of gap 306 is deteriorated with the number of times of use of the lamp 306.
Meanwhile, there also occurs a problem where cost is increased since the two transformer windings 302b and 302c are used for starting circuit 301 and direct current power source circuit 307.